JP6155434B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus provided with a charging unit of a proximity charging method to which a charging voltage obtained by superimposing an AC voltage on a DC voltage is applied.

  In recent years, as a charging method in an image forming apparatus, a proximity charging method is becoming mainstream from the viewpoint of reducing ozone or the like. In the proximity charging system, for example, a roller, brush, or blade-shaped charging unit is disposed in proximity to or in contact with the surface of a photosensitive drum as a typical example of an image carrier.

  The charging means is applied with a charging voltage obtained by superimposing an AC voltage having a predetermined peak-to-peak voltage value Vpp1 on a DC voltage so that the surface of the image carrier is uniformly charged in a predetermined process such as a printing process. (For example, see Patent Document 1).

  In Patent Document 1, in order to generate a stable discharge between the charging unit and the image carrier, the power source unit performs only charge transfer from the charging unit to the image carrier (that is, charge transfer in one direction). AC voltages having different detection peak-to-peak voltages Vpp0 are sequentially applied to the charging means in the positive discharge region where the phenomenon occurs and in the reverse discharge region where bidirectional charge transfer occurs alternately. The control means derives approximate functions f1 (Vpp) and f2 (Vpp) of the alternating current value Iac with respect to the peak-to-peak voltage value Vpp for each of the positive discharge region and the reverse discharge region, and then the difference indicating the discharge current amount ΔIac. A discharge current amount ΔIac and a predetermined peak-to-peak voltage value Vpp1 at which the function (ΔIac (= f2 (Vpp) −f1 (Vpp))) is a predetermined value D are derived.

JP 2001-201920 A

  In Patent Document 1, the predetermined value D is constant regardless of the degree of wear of the image carrier due to repeated printing or the like. However, as a result of experiments by the present inventors, it has been found that the optimum predetermined value D varies depending on the degree of wear. Specifically, it has been found that the optimum constant D decreases with the progress of wear. Therefore, the peak-to-peak voltage value Vpp1 that was optimum at the beginning of the life of the image carrier is larger than necessary at the end of the life (that is, it is not the optimum value), so that the image carrier is greatly damaged. It will be. As a result, wear is promoted, and the life of the image carrier becomes shorter than the designed value.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of suppressing the shortening of the life of an image carrier.

  One aspect of the present invention is an image forming apparatus that prints an image on a medium when a sheet is passed, and includes an image carrier, a charging unit that is arranged in proximity to the image carrier, and a predetermined peak interval during a predetermined process. A power supply means for applying a first charging voltage including an AC voltage having a voltage value to the charging means, wherein when determining the predetermined peak-to-peak voltage value, there are a plurality of second charging voltages each including an AC voltage. Between the positive discharge area where only charge transfer from the charging means to the image carrier and the reverse discharge area where the charge moves in both directions between the image carrier and the charging means are between different peaks. A power supply means for sequentially applying a plurality of second charging voltages to each of the AC voltages, and an AC current value flowing through the charging means during application of each of the plurality of second charging voltages. Detection A first approximation function and a second approximation function indicating an alternating current value with respect to a peak-to-peak voltage value for the forward discharge region and the reverse discharge region based on each of the current detection unit and the alternating current value detected by the current detection unit. An approximate function is derived, and in the difference function indicating the difference value between the derived first approximate function and the second approximate function, a peak-to-peak voltage value at which a change amount of the difference value per unit peak-to-peak voltage is a predetermined value The image forming apparatus includes control means for setting the peak-to-peak voltage.

  According to the above aspect, an object is to provide an image forming apparatus capable of suppressing the shortening of the life of the image carrier.

1 is a schematic diagram illustrating a schematic configuration of an image forming apparatus. 1 is a schematic diagram illustrating a configuration of a main part of an image forming apparatus according to an embodiment. It is a flowchart which shows the first half of the process sequence at the time of determination of the voltage value between peaks. It is a flowchart which shows the process sequence of the part following FIG. 3A. It is a flowchart which shows a detailed process sequence of S215 of FIG. 3B. It is a figure which shows the specific processing content of FIG. A graph showing the difference in the current value detected by the current detecting means depending on the life of the photosensitive drum (upper stage) and a graph showing that the optimum peak-to-peak voltage value is different depending on the life of the photosensitive drum (lower stage). is there. It is a graph which shows the change of the peak-to-peak voltage value (the derived value of this embodiment, the derived value of patent document 1, and the optimal value) with respect to the number of printed sheets. It is a graph which shows the change of the optimal peak-to-peak voltage value by machine temperature. It is a flowchart which shows with the detailed process sequence of S215 of FIG. 3B which concerns on a 1st modification. It is a flowchart which shows with the detailed process sequence of S215 of FIG. 3B which concerns on a 2nd modification.

  Hereinafter, the image forming apparatus will be described in detail with reference to the drawings.

<< First column: Definition >>
Some figures show an x-axis, a y-axis and a z-axis that are orthogonal to each other. The x axis and the z axis indicate the left and right direction and the up and down direction of the image forming apparatus 1. The y-axis indicates the front-rear direction of the image forming apparatus 1.

<< Second Column: Overall Configuration / Printing Process of Image Forming Apparatus According to Embodiment >>
1 and 2, an image forming apparatus 1 is, for example, a copying machine, a printer, a facsimile machine, or a multifunction machine having these functions. Various types of images (typically, by a known electrophotographic system and tandem system). Specifically, a full-color image or a monochrome image) is printed on a sheet-like print medium (paper or OHP sheet) M. Therefore, the image forming apparatus 1 includes a yellow (Y), magenta (M), cyan (C), and black (K) image forming unit 2, an intermediate transfer belt 3, a secondary transfer roller 4, and a power source. Means 10, control means 11, current detection means 12, and temperature and humidity detection means 13 are provided.

  The image forming units 2 for the respective colors are juxtaposed in the left-right direction, for example, and include corresponding photosensitive drums 5.

  Each photoconductor drum 5 has, for example, a cylindrical shape extending in the front-rear direction, and rotates about its own axis, for example, in the direction of arrow α.

  As clearly shown in FIG. 2, at least the charging unit 6, the developing unit 8, the primary transfer roller 9, and the like around the above photosensitive drums 5 from the upstream side to the downstream side in the rotation direction α. Is placed. Note that FIG. 2 representatively shows a certain color photosensitive drum 5 and its periphery.

  Each charging unit 6 is typically a charging roller that extends in the front-rear direction, and is a charging roller that is disposed close to or in contact with the peripheral surface of the photosensitive drum 5. Each charging means 6 uniformly charges the peripheral surface of the rotating photosensitive drum 5 with the charging voltage Vg.

  In order to apply the charging voltage Vg to each charging means 6, the image forming apparatus 1 is provided with a power supply means 10 for each color, for example, as clearly shown in FIG. Each power supply means 10 includes a set of a DC power supply circuit 101 and an AC power supply circuit 102. Note that FIG. 2 representatively shows a single color power supply means 10.

  Each DC power supply circuit 101 outputs a predetermined DC voltage Vdc under the control of the control means 11. In the present embodiment, since it is not important to change the DC voltage Vdc for each color, in the following description, the DC voltage Vdc is described as the same value for each color for convenience.

  Each AC power supply circuit 102 is composed of, for example, an AC transformer, and outputs an AC voltage Vac having a peak-to-peak voltage value Vpp selected or determined by the control means 11. Note that, from the same viewpoint as the DC voltage Vdc, the description will be continued assuming that each AC voltage Vac has the same peak-to-peak voltage value Vpp.

  The output terminal of each AC power supply circuit 102 is connected to the output terminal of the corresponding DC power supply circuit 101, whereby a charging voltage Vg in which the AC voltage Vac is superimposed on the DC voltage Vdc is generated, and the charging means 6 for the corresponding color. To be applied.

  An exposure device 7 is provided below each photosensitive drum 5. Each exposure device 7 irradiates a light beam B based on the image data to an exposure area immediately downstream of the charging area of the photosensitive drum 5, thereby forming an electrostatic latent image of a corresponding color.

  Each developing means 8 forms a corresponding color toner image by supplying a corresponding color developer to the developing area immediately downstream of the exposure area of the corresponding photosensitive drum 5.

  The intermediate transfer belt 3 is looped over the outer peripheral surfaces of at least two rollers arranged in the left-right direction, for example, and rotates in the direction indicated by the arrow β, for example. For example, the outer peripheral surface of the intermediate transfer belt 3 is in contact with the upper end of each photosensitive drum 5.

  Each primary transfer roller 9 opposes the corresponding photosensitive drum 5 with the intermediate transfer belt 3 interposed therebetween, and presses the intermediate transfer belt 3 from above so that the primary transfer roller 9 is primary between the photosensitive drum 5 and the intermediate transfer belt 3. A transfer nip 91 is formed. A primary transfer bias voltage is applied to each primary transfer roller 9 during the printing process. As a result, the toner image on the photosensitive drum 5 is transferred to the rotating intermediate transfer belt 3 at the corresponding primary transfer nip 91. Is done.

  The secondary transfer roller 4 is configured to be rotatable about its own axis. A secondary transfer bias voltage is applied to the secondary transfer roller 4 during the printing process. The secondary transfer roller 4 presses the outer peripheral surface of the intermediate transfer belt 3 in the vicinity of the right end of the intermediate transfer belt 3, for example, so that the secondary transfer nip is brought into contact with the secondary transfer roller 4 and the intermediate transfer belt 3. 41 is formed. The secondary transfer nip 41 is fed with the print medium M during the printing process.

  Since the secondary transfer bias voltage is applied to the secondary transfer roller 4 while the printing medium M is passing through the secondary transfer nip 41 (that is, during paper passing), the toner image carried on the intermediate transfer belt 3 is transferred to the secondary transfer roller 4. It moves to the printing medium M and is transferred. The print medium M passes through the secondary transfer nip 41, passes through a known fixing device, and is then discharged as a printed matter to the tray.

  As clearly shown in FIG. 2, the control unit 11 includes, for example, a ROM 111, a CPU 112 as an example of a processing unit, an SRAM 113, and an NVRAM 114 as an example of a storage unit. The CPU 112 controls various processes by executing a control program stored in advance in the ROM 111 while using the SRAM 113 as a work area. This embodiment is particularly related to the following four processes (that is, printing, image stabilization, forced toner replenishment, and TCR adjustment). In the following four processes, since the photosensitive drum 5 needs to be charged, the charging means 6 includes a charging voltage Vg including the AC voltage Vac having a predetermined peak-to-peak voltage value Vpp1 (hereinafter referred to as a first charging voltage Vg1). ) Is applied.

(1) Printing: Printing an image on the printing medium M (2) Image stabilization: Controlling the toner density to a target value based on the density of a known pattern image (3) Forced toner supply: Forcing the developing means (4) TCR adjustment: controlling the toner to carrier ratio to the target value

  In addition, the CPU 112 performs a “determining process of a predetermined peak-to-peak voltage value” (details will be described later) to derive a predetermined peak-to-peak voltage value Vpp1 to be used in each process described above.

  The current detection means 12 flows to the charging means 6 via the corresponding photosensitive drum 5 when a charging voltage Vg2 described later is applied to each charging means 6 in at least a predetermined peak-to-peak voltage value derivation process. The alternating current value Iac is detected and output to the CPU 112.

  The temperature / humidity detection means 13 includes a temperature sensor 131 and a humidity sensor 132. The temperature sensor 131 detects the temperature in the image forming apparatus 1 (that is, the in-machine temperature) St and outputs it to the CPU 112. On the other hand, the humidity sensor 132 detects the relative humidity (hereinafter referred to as “in-machine humidity”) Sh in the image forming apparatus 1 and outputs it to the CPU 112.

<< Third column: Charging voltage determination process (including predetermined peak-to-peak voltage value derivation process) >>
Next, the operation of the image forming apparatus 1 will be described with reference to FIGS. 3A to 7.
In FIG. 3A, when determining the peak-to-peak voltage in the above four processes, the CPU 112 detects the temperature and humidity detection means 13 in a state where the print medium M is not conveyed into the image forming apparatus 1 (that is, in a non-sheet passing state). Then, the current in-machine temperature St and in-machine humidity Sh are acquired (S21).

  Next, the CPU 112 acquires the environmental step corresponding to the in-machine temperature St and the in-machine humidity Sh obtained in S21 from the environment step table T1 previously stored in the ROM 111 or the NVRAM 114 (S22). As shown in Table 1 below, the table T1 describes an environmental step that is an index indicating the magnitude of absolute humidity for each combination of in-machine temperature and in-machine humidity. This table T1 is created in advance by experiments or the like at the manufacturing stage or development stage of the image forming apparatus 1. This also applies to other tables. In this embodiment, the environmental steps are divided into sixteen stages, the environmental steps 1 to 3 are a low temperature and low humidity environment (so-called LL environment), the environmental steps 4 to 7 are a normal temperature and normal humidity environment (so-called NN environment), Environmental steps 8 to 12 indicate a slightly high temperature and high humidity environment, and environmental steps 13 to 16 indicate a high temperature and high humidity environment (so-called HH environment).

  Next, the CPU 112 sets a set of peak-to-peak voltage values Vpp (hereinafter referred to as a detection peak-to-peak voltage Vpp0) corresponding to the environmental step obtained in S22 from the peak-to-peak voltage value table T2 previously stored in the NVRAM 114 or the like. One is selected (S23). In the table T2, as shown in Table 2 below, a set composed of a plurality of different peak-to-peak voltage values Vpp0 for detection (in the present embodiment, eight cases are described) is described for each range of the environmental step. The Each set includes at least two detection peak-to-peak voltage values Vpp0 for each of the positive discharge region and the reverse discharge region (in this embodiment, four cases are described). In this embodiment, the peak-to-peak voltage value Vpp (described later) is less than 2 × Vth (see FIG. 5), and when the charging voltage Vg is applied to the charging unit 6, the charging unit 6 transfers to the photosensitive drum 5. The region of the peak-to-peak voltage Vpp where only the charge transfer (charge transfer in one direction) occurs is referred to as a positive discharge region. On the contrary, a region that is 2 × Vth or more (see FIG. 5) and in which charge transfer occurs bi-directionally between the photosensitive drum 5 and the charging unit 6 is referred to as a reverse discharge region. Here, Vth is a voltage at which charging of the photosensitive drum 5 starts.

  For example, the set A of the detected peak-to-peak voltage value Vpp0 is assigned to the environmental steps 1 to 3, and the set A is included in the reverse discharge region, 600V, 700V, 800V, and 900V included in the normal discharge region. It consists of 1850V, 1950V, 2050V and 2150V. The combinations B, C, and D of the peak-to-peak voltage values Vpp0 for detection as shown in Table 2 are assigned to the environmental steps 4 to 7, 8 to 12, and 13 to 16.

  Next, the CPU 112 initializes the first counter value n to 1 (S24), and acquires the detection peak-to-peak voltage value Vpp0 corresponding to the current first counter value n in the group selected in S23 (S25).

  The CPU 112 sets the peak-to-peak voltage value Vpp of the AC voltage Vac to be output from the AC power supply circuit 102 for each color to the detection peak-to-peak voltage value Vpp0 acquired in S25. Further, the CPU 112 sets the DC voltage Vdc to be output from the DC power supply circuit 101 for each color to a predetermined value (S26).

  As a result of S26, the above-mentioned second charging voltage Vg2 is applied from the power supply means 10 to the charging means 6 of each color. When the AC voltage Vac of each AC power supply circuit 102 is stabilized (S27), the CPU 112 initializes the second counter value m to 1 (FIG. 3B; S28). Next, the CPU 112 acquires the AC current value Iac from the current detection means 12 for each color, and stores the acquired AC current value Iac in the SRAM 113 for each color (S29).

  Next, the CPU 112 determines whether or not the second counter value m is y (S210). Here, y is the number of samplings per rotation of each photosensitive drum 5, and is a natural number of 1 or more. If it is N in S210, the CPU 112 increments the second counter value m by 1 (S211), and performs S29.

  By repeating S28 to S211, the SRAM 113 stores the AC current values Iac of the respective colors measured at y different locations in the circumferential direction during one rotation of the photosensitive drums 5 of the respective colors. Retained. If it is Y in S210, the CPU 112 derives an average value of the y AC current values Iac and stores it in the SRAM 113 (S212).

  In the present embodiment, in order to smooth the variation in the film thickness of the photosensitive drum 5 through S28 to S212, the CPU 112 has a plurality of different locations in the circumferential direction while each photosensitive drum 5 rotates once. It is preferable to take the average value of the y AC current values Iac obtained in step (b).

  Next, the CPU 112 determines whether or not the processing of S25 to S212 has been performed for all the peak-to-peak voltage values Vpp0 for detection of the set selected in S23 by determining whether or not the first counter value n is 8. (S213). If it is N in S213, the CPU 112 increments the first counter value n by 1 (S214), and performs S25 of FIG. 3A.

  As a result of S25 to S214, the alternating current value Iac (average value) flowing through the charging means 6 when four second charging voltages Vg2 are sequentially applied to the SRAM 113 in each of the positive discharge region and the reverse discharge region. Can be obtained eight by one. The CPU 112 holds eight combinations of the detection peak-to-peak voltage value Vpp0 used in S26 and the alternating current value (average value) Iac obtained in S212 in the SRAM 113 for each color. Hereafter, the combination of the detected peak-to-peak voltage value Vpp0 and the alternating current value Iac held in the SRAM 113 will be generically expressed as (Vpp0, Iac). In addition, when any of n = 1 to 8 is expressed individually, it is expressed as (Vpp0j, Iacj). Here, j is a natural number of 1, 2,.

  If Y is determined in S213, the CPU 112 derives a predetermined peak-to-peak voltage value Vpp1 based on (Vpp0, Iac) for each color (S215).

Here, with reference to FIG. 4 and FIG. 5, the peak voltage value deriving process will be described in detail.
First, in FIG. 4, the CPU 112 selects four sets (Vpp, Iac) belonging to the positive discharge region for each color, and linearly approximates these four sets of data by the least square method or the like. As a result, the CPU 112 has a straight line L1 (see the upper part of FIG. 5) approximating the characteristic of the alternating current value Iac (hereinafter referred to as Vpp-Iac characteristic) with respect to the peak-to-peak voltage value Vpp in the positive discharge region, that is, the first approximation function. Iac = f1 (Vpp) (where Vpp <2 × Vth) is obtained (S31).

  Next, for each color, the CPU 112 approximates four sets of (Vpp, Iac) belonging to the reverse discharge region by a curve and approximates a Vpp-Iac characteristic in the reverse discharge region (see the upper part of FIG. 5). That is, the second approximate function Iac = f2 (Vpp) (where 2 × Vth ≦ Vpp) is obtained (S32). The reason why the curve is approximated in S32 is that the actual Vpp-Iac characteristic in the reverse discharge region is not a straight line but close to a curve.

  Next, the CPU 112 derives a function approximating the discharge current amount ΔIac with respect to the peak-to-peak voltage value Vpp (S33). As a specific example, f2 (Vpp) -f1 (Vpp) obtained by subtracting the first approximation function from the second approximation function is derived as a difference function indicating ΔIac (that is, a difference value).

  Next, as shown in the lower part of FIG. 5, the CPU 112 determines that the difference value (discharge current amount) ΔIac per unit peak voltage ΔIac change amount (that is, the differential value (dΔIac / dVpp)) is a predetermined value k in the difference function. A discharge current amount ΔIac is derived (S34). In the present embodiment, the amount of change in the discharge current amount ΔIac is an increase amount. Thereafter, the CPU 112 derives the peak-to-peak voltage value Vpp corresponding to the derived discharge current amount ΔIac as the predetermined peak-to-peak voltage value Vpp1 in the difference function ΔIac (S35).

  Here, the predetermined value k is a constant value regardless of the number of printed sheets, and is defined in advance at the design and development stage of the image forming apparatus 1 as follows, for example. With the new photoconductor drum 5 mounted, the printing process is tried using the charging voltage Vg on which the alternating voltage Vac having various peak-to-peak voltage values Vpp is superimposed. In this trial, other conditions are met. By the above trial, the characteristic curve of Vpp−ΔIac is first actually measured. Further, the peak-to-peak voltage is such that the image quality of the printed matter obtained by the trial and the damage to the photosensitive drum 5 are judged by visual observation, for example, a high-quality printed matter is obtained, and the damage to the photosensitive drum 5 is very small. The value Vpp is selected as the optimum value. The predetermined value k is the slope of the tangent line at the peak-to-peak voltage value Vpp at which a high image quality is obtained in the Vpp-ΔIac characteristic curve (that is, the amount of change in the difference value (discharge current amount) ΔIac per unit peak-to-peak voltage). ) And has a value in the range of approximately 1/5 to 1/4.

  When S34 ends, the CPU 112 ends the process of FIG. 4 (that is, ends S215 of FIG. 3B) and performs S216 of FIG. 3B. That is, the CPU 112 sets the peak-to-peak voltage value Vpp of the AC voltage Vac to be output from each AC power supply circuit 102 to the predetermined peak-to-peak voltage value Vpp1 derived in S215, and the DC to be output from each DC power supply circuit 101. The voltage Vdc is set to a predetermined value. As a result, the first charging voltage Vg1 is applied from the power supply means 10 to each charging means 6, and each photosensitive drum 5 is charged (S216).

<< Column 4: Actions and effects of image forming apparatus >>
In general, the film thickness of the photosensitive drum decreases with repeated printing, and therefore decreases as the number of printed sheets increases. Accordingly, since the resistance value becomes relatively small at the end of the life of the photosensitive drum, even when the same peak-to-peak voltage Vpp is applied, a larger current Iac flows in the photosensitive drum 5 at the end of the life (the upper part of FIG. 6). See). Accordingly, the peak-to-peak voltage value Vpp, which is almost the same as the optimum value at the beginning of the life, is not always the optimum value at the end of the life. Specifically, the optimum value at the end of life is smaller than the optimum value at the beginning of life. According to an experiment conducted by the present inventors, if the optimum value is 2150 V at the beginning of the life, for example, it drops to 1480 V at the end of the life (see the lower part of FIG. 6). Here, the optimum value is, for example, a value that is appropriately determined by visual observation in the same manner as described above, and that provides a high-quality printed matter.

  By the way, as described above, Patent Document 1 derives a predetermined peak-to-peak voltage value Vpp1 at which the discharge current value ΔIac (= f2 (Vpp) −f1 (Vpp)) becomes a constant value D. Here, as shown in the lower part of FIG. 6, assuming that the constant value D is 210, for example, at the beginning of the life of the photosensitive drum, a peak-to-peak voltage value Vpp1 that is almost the same as the optimum value (indicated by a circle at about 2150 V) is derived. However, at the end of the life, an inappropriate value (indicated by Δ at about 1740V) that is considerably larger than the optimum value (indicated by ○ at about 1480V) is derived. As described above, in the method disclosed in Patent Document 1, the predetermined peak-to-peak voltage value Vpp1 that is close to the optimum value according to the life of the photosensitive drum 5 may not be derived.

  On the other hand, according to the present image forming apparatus 1, the peak-to-peak voltage value such that the change amount (dΔIac / dVpp) of the difference value (discharge current amount) ΔIac per unit peak-to-peak voltage becomes the predetermined value k in the difference function. Vpp is derived as a predetermined peak-to-peak voltage value Vpp1. The discharge current amount ΔIac increases with an increase in the peak-to-peak voltage value Vpp regardless of whether the photosensitive drum 5 is at the beginning or end of its life. However, regarding the amount of change (dΔIac / dVpp), the end of life starts to increase from the smaller peak-to-peak voltage value Vpp.

  In the present embodiment, based on the above tendency, the predetermined peak-to-peak voltage value Vpp1 is determined so that the amount of change (dΔIac / dVpp) becomes the predetermined value k (S34 in FIG. 4). / 4, for example, not only the peak-to-peak voltage value Vpp1 close to the optimum value (about 2150V) is derived at the beginning of the life of the photosensitive drum, but also between the peaks near the optimum value (about 1480V) at the end of the life. A voltage value Vpp1 (about 1500 V) is derived. In other words, in the present embodiment, a predetermined peak-to-peak voltage value Vpp1 having a difference of only 20 V with respect to the optimum value obtained from actual measurement is derived even at the end of the life (see Table 3).

  The inventor further measured the change in the peak-to-peak voltage value Vpp with respect to the number of printed sheets. The result is shown in FIG. In FIG. 7, ◯ represents an optimum value obtained by visual observation, □ represents a predetermined peak-to-peak voltage value Vpp1 obtained in the present embodiment, and Δ represents a predetermined peak-to-peak voltage value Vpp1 obtained in Patent Document 1. Indicates.

  As described above, the optimum value decreases as the number of printed sheets increases. This point is the same in this embodiment and Patent Document 1. However, the peak-to-peak voltage value Vpp in Patent Document 1 deviates from the optimum value as the number of printed sheets increases. On the other hand, the predetermined peak-to-peak voltage value Vpp1 of this embodiment closely follows the optimum value and follows, regardless of the number of printed sheets.

  The present inventor further measured the amount of wear of the photosensitive drum 5 after printing 100k sheets in the image forming apparatuses of Patent Document 1 and the present embodiment. Here, the design target value of the amount of wear after printing 100k sheets was 15 μm. As a result of measurement, the amount of wear was 14.8 μm in the image forming apparatus of Patent Document 1, but the amount of wear was 11.2 μm in the image forming apparatus 1 of the present embodiment (see Table 4 below). .

  As described above, by determining the predetermined peak-to-peak voltage value Vpp1 so that the change amount (dΔIac / dVpp) becomes the predetermined value k regardless of the number of printed sheets, an image capable of suppressing the shortening of the life of the photosensitive drum 5 is achieved. A forming apparatus can be provided.

<< 5th column: 1st modification >>
As is well known, each charging means 6 is difficult to discharge in a low temperature environment, and the approximate functions f1 (Vpp) and f2 (Vpp) when the in-machine temperature St is low are on the constant current side as compared with those at high temperatures. Along with the shift, the increase amount of Iac relative to the unit peak-to-peak voltage is also reduced. The same applies to the difference function ΔIac. It is also known that the optimum value is shifted to the high voltage side.

  As shown in the lower part of FIG. 8, when the in-machine temperature St is high (for example, at 30 ° C.), it is assumed that the optimum value (see ○) is about 1260V. Furthermore, it is assumed that the predetermined value k is set to about ¼ so that the above embodiment is optimized at a high temperature and about 1275 V is derived as a reference peak-to-peak voltage Vpp1 (see □) at a high temperature. However, under a low temperature such as 10 ° C., the optimum value (see ○) is about 1480 V. When the predetermined value k is set to about ¼, in the above embodiment, the optimum value is lower than the optimum value. It was found that a predetermined peak-to-peak voltage value Vpp1 (about 1603V, see □) that is about 123V higher is derived. When such a high predetermined peak-to-peak voltage value Vpp1 is applied to the charging means 6, the wear of the photosensitive drum 5 is promoted.

  In view of the above problems, an object of the present modification is to provide an image forming apparatus that can set an appropriate predetermined value k according to the in-machine temperature St.

  The first modification is different from the above embodiment in that the process of FIG. 9 is performed instead of the process of FIG. Since there is no difference other than the above, in this modification, the same code | symbol is attached | subjected to the structure and process corresponding to the said embodiment, and each description is abbreviate | omitted.

  In FIG. 9, after S32 (details are described above), the CPU 112 acquires the current in-machine temperature St from the environment detection means 13 (S41). Next, the CPU 112 acquires a predetermined value k corresponding to the in-machine temperature St obtained in S41 from a predetermined value table T1 previously stored in the ROM 111 or the NVRAM 114 (S42). In the table T1, a predetermined value k is described for each temperature range as shown in Table 5 below. In this embodiment, the temperature range is divided into three stages of less than 15 ° C., 15 ° C. or more and less than 25 ° C., and 25 ° C. or more. As the predetermined value k, 1/5, 1/4. 5 and 1/4 are assigned to less than 15 ° C, 15 ° C to less than 25 ° C and 25 ° C or more. Such a predetermined value k is appropriately determined in advance in an experiment or the like at the design and development stage of the image forming apparatus 1A so that a reference peak voltage value Vpp1 substantially the same as the optimum value can be derived in the corresponding temperature range.

  Next, the CPU 112 performs S33 (details are described above) using the predetermined value k acquired in S42.

  With the above processing, it is possible to provide the image forming apparatus 1 capable of setting an appropriate predetermined value k according to the in-machine temperature St. Specifically, according to the actual measurement result of the present inventors, as shown in Table 6 below, when the in-machine temperature St is 30 ° C., the optimum value is about 1425V. Further, the inventor of the present invention optimizes at 30 ° C. in the above embodiment and sets the predetermined value k to ¼. As a result, at 30 ° C., the image forming apparatus 1 described in the embodiment derived approximately 1434 V as the reference peak-to-peak voltage Vpp1 close to the optimum value. However, for example, at 10 ° C., the optimum value is about 1960 V. However, the image forming apparatus 1 according to the embodiment has a predetermined peak that is about 106 V higher than the optimum value at a predetermined value k of about ¼. An inter-voltage value Vpp1 (about 2066V) was derived. Therefore, in this modification, the predetermined value k is set to 1/5 at less than 15 ° C., so that at 10 ° C., the image forming apparatus 1 described in this modification uses the reference peak-to-peak voltage Vpp 1 closer to the optimum value. About 1981V could be derived.

In addition, the present inventor measured the amount of wear of the photosensitive drum 5 after printing under the following durability conditions (that is, after the durability test) for the method of Patent Document 1, the above embodiment, and the first modification. Then, the image quality of the printed matter was measured visually, and the actual measurement results are summarized in Table 7 below for comparison. The conditions of the durability test were as follows.
Size of print medium M: A4 landscape Printed toner image: Chart with 5% image coverage Number of prints: 100k

  First, the present inventor prepared an image forming apparatus disclosed in Patent Document 1, in which an optimum predetermined value D was set at 30 ° C. Thereafter, the endurance test was conducted with this image forming apparatus under an in-machine temperature St of 10 ° C. In this case, since the predetermined peak-to-peak voltage value Vpp1 is higher than the optimum value, the amount of wear of the photosensitive drum 5 is 15 μm or more after the durability test, and is larger than the target value (15 μm) at the time of design. The evaluation of the amount of wear was NG (No Good). At 20 ° C., the amount of wear was 14 μm or more and less than 15 μm after the durability test, and the evaluation of the amount of wear was Ave. (Average). Moreover, at 30 ° C., the amount of wear was less than 14 μm, and the evaluation of the amount of wear was G (Good). Further, in this case, regardless of the temperature, deterioration in image quality due to poor charging could not be visually confirmed, and the evaluation regarding image quality was G under all temperature conditions.

  In addition, the present inventor conducted the above durability test at an in-machine temperature St of 10 ° C. in the image forming apparatus of Patent Document 1 in which an optimal predetermined value D was set at 20 ° C. And confirmed the image quality. Also in this case, since the predetermined peak-to-peak voltage value Vpp1 is derived high, the evaluation of the amount of wear was NG. Further, under 30 ° C., a predetermined peak-to-peak voltage value Vpp1 is derived low, and charging failure occurs. Therefore, the image quality evaluation is NG. Further, since the toner is supplied to the non-image area due to charging failure and the amount of wear is accelerated, the amount of wear is also evaluated as NG.

  In addition, the present inventor conducted the above durability test at an in-machine temperature St of 30 ° C. in the image forming apparatus of Patent Document 1 in which the optimum predetermined value D was set at 10 ° C. The amount of wear and image quality were confirmed. For the same reason as described above, the evaluation of the amount of wear and the image quality was NG. Also, at 20 ° C., the evaluation of the amount of wear and the image quality is Ave. Met.

  As described above, according to the image forming apparatus described in Patent Document 1, the photosensitive drum 5 is easily worn out by repeated printing. In addition, depending on the in-machine temperature, the amount of wear of the photosensitive drum 5 is remarkably increased and the image quality is also affected.

  On the other hand, the image forming apparatus 1 according to the embodiment derives the discharge current amount ΔIac at which the differential value (dΔIac / dVpp) becomes the predetermined value k even when the image forming apparatus 1 is optimized at 30 ° C., for example. The predetermined peak-to-peak voltage value Vpp1 follows the optimum value. As a result, even when the durability test is performed at temperatures of 10 ° C. and 20 ° C., the evaluation of the amount of wear and the image quality is G.

  Further, in the image forming apparatus 1 according to the first modification, since the predetermined peak-to-peak voltage value Vpp1 is derived so as to follow the fluctuation of the optimum value due to the temperature change, the amount of wear is evaluated in Excel. (Excellent), and the evaluation of the image quality was G.

<Sixth column: second modification>
Also, in S29 of FIG. 3B, an abnormal AC current value Iac may be detected due to noise. As a result, there is a possibility that an unexpected discharge current amount ΔIac and thus an unexpected predetermined peak-to-peak voltage value Vpp1 may be derived in S34 and S35 of FIG. Specifically, if the reference peak voltage value Vpp1 is inappropriately large, the wear of the photosensitive drum 5 is accelerated, and if it is inappropriately small, the image quality of the printed matter is affected.

  Therefore, in this modification, the process shown in FIG. 10 is performed instead of FIG. As shown in FIG. 10, the CPU 112 determines whether or not the discharge current amount ΔIac derived in S34 is outside the assumed range between S34 and S35 described above (S51). In the present modification, for example, if it is equal to or higher than a predetermined upper limit (for example, 250 μA) or lower than a predetermined lower limit (for example, 100 μA), it is determined that it is not expected.

  If it is N in S51, the CPU 112 derives a peak-to-peak voltage value Vpp at which the discharge current amount ΔIac becomes a predetermined fixed value D as a predetermined peak-to-peak voltage value Vpp1 in the difference function ΔIac (S52).

  On the other hand, if it is Y in S51, the CPU 112 performs S35. When one of S35 and S52 ends, the CPU 112 exits the process of FIG. 10 and performs S216 of FIG. 3B. At this time, the predetermined peak-to-peak voltage value Vpp1 derived in any one of S35 and S52 is used.

<5th column: Appendix>
In the description of the above embodiment, the image forming apparatus 1 includes the current detection unit 12 for each color. However, the present invention is not limited to this, and the current detection means 12 may be provided only in the specific one to three color charging means 6 representing all colors.

In the above embodiment, the difference function is f2 (Vpp) -f1 (Vpp). However, the present invention is not limited to this, and the difference function may be f1 (Vpp) -f2 (Vpp). In this case, the change amount in the difference function is the decrease amount.

  The image forming apparatus according to the present invention can suppress the shortening of the life of the image carrier, and is suitable for a facsimile machine, a copier, a printer, and a multifunction machine having these functions regardless of whether it is a color machine or a monochrome machine. .

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 5 Photosensitive drum (image carrier)
6 Charging means 10 Power supply means 11 Control means 12 Current detection means 13 Temperature / humidity detection means

Claims (5)

An image forming apparatus that prints an image on a medium when passing paper,
An image carrier;
Charging means disposed in proximity to the image carrier;
A power supply means for applying a first charging voltage including an AC voltage having a predetermined peak-to-peak voltage value to the charging means during a predetermined process, wherein the AC voltage is determined when determining the predetermined peak-to-peak voltage value, respectively. A plurality of second charging voltages including a positive discharge region in which only charge transfer from the charging means to the image carrier and a reverse movement in which charge moves bidirectionally between the image carrier and the charging means. A power supply means for sequentially applying a plurality of second charging voltages of the AC voltage having different peak-to-peak voltages in each of the discharge regions to the charging means;
Current detecting means for detecting an alternating current value flowing through the charging means during application of each of the plurality of second charging voltages;
Based on each of the alternating current values detected by the current detection means, for the positive discharge region and the reverse discharge region, to derive a first approximate function and a second approximate function indicating the alternating current value with respect to the peak-to-peak voltage value, In the difference function indicating the difference value between the derived first approximation function and the second approximation function, the peak-to-peak voltage value at which the change amount of the difference value per unit peak-to-peak voltage becomes a predetermined value is set as the predetermined peak-to-peak voltage. An image forming apparatus comprising a control unit.   The image forming apparatus according to claim 1, wherein the predetermined value is constant regardless of the number of printed sheets.   The image forming apparatus according to claim 1, wherein the control unit approximates an alternating current value with respect to a peak-to-peak voltage value with a straight line and a curve for the positive discharge region and the reverse discharge region. An environment detection means for detecting the temperature in the image forming apparatus,
The control means includes
Determining the predetermined value based on the temperature detected by the environment detection means;
The image forming apparatus according to claim 1, wherein the peak-to-peak voltage value at which the amount of change in the difference function is a predetermined value determined is the predetermined peak-to-peak voltage. The control means includes
In the difference function, the difference value at which the amount of change per unit peak-to-peak voltage becomes the predetermined value is obtained,
The peak-to-peak voltage value at which the amount of change becomes a predetermined fixed value is determined as the predetermined peak-to-peak voltage when the obtained difference value is an unexpected value. Image forming apparatus.

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